Polymerization inhibition by alkyl-hydroxylamine salts



United States Patent 13 Claims. (Cl. '260'-666.5)

ABSTRACT OF THE DISCLOSURE Styrene is stabilized against polymerizationby .001- 5.0% dosage with diethylhydroxylamine salt of hydroxycarboxylicand other carboxylic acids, including lactate, tartrate, and citrate.

This application is a division of application Ser. No. 319,961, filedOct. 30, 1963, now Patent 3,290,364.

This invention relates to the stabilization of aromatic vinyl compoundswith a novel class of compounds and to the stabilized compositionsobtained.

It is known that aromatic vinyl compounds such as styrene,divinylbenzene, alpha methyl styrene and the like, tend to polymerize onstanding, and numerous inhibitors have been employed heretofore toprevent this premature polymerization. Among the inhibitors previouslyused have been N,N-dialkylhydroxylamines and their simple inorganicsalts, as, for example, the hydrochloride and sulfate, and suchtechnology is taught in US. Patents 2,965,685; 2,225,471, and 2,318,212.It has now been found, however, that a surprising improvement in thetechniques of stabilizing aromatic vinyl compounds can be achieved byemploying as stabilizers N,N-di-lower alkylhydroxylamine salts ofhydroxy acids and polycarboxylic acids which have the structural formulawhere R is a member selected from the group of methyl and carboxyl, R isa member selected from the group of hydrogen and hydroxyl, R is a memberselected from the group of hydrogen and carboxyl, n is an integer from 0to 8 and with the proviso that when R is methyl at least one R ishydroxyl.

It is surprising that the N,N-dialkylhydroxylamine salts of acids of theabove structure are operable as stabilizers. Aromatic carboxylic acidsalts (e.g. benzoates, salicylates, etc.) are inoperable as stabilizersfor aromatic vinyl compounds even though they have inherent mutualsolubility and compatibility with aromatic vinyl compounds. Furthermore,not all polycarboxylic acid salts of N,N-dialkylhydroxylamines show thesuperior stabilizing effects obtained with the acids of this invention.The oxalate salts of N,N-di-lower alkylhydroxylamines, for example, areequivalent to the use of the N,N-di-lower alkylhydroxylamines alone andfall short of the inhibiting activity of the acid salts given above.Also, the olefinic carboxylic acid salts (e.g. maleic acid salts) arenot operable as stabilizers for vinyl monomers.

It will be understood in discussing this invention that 3,392,204Patented July 9, 1968 "ice the acid salts used may be those of anyN,N-di-lower alkylhydroxylamine, i.e., a hydroxylamine having attachedto the nitrogen atom two alkyl groups containing from one to 6 carbonatoms in each alkyl group; e.g., N,N- dimethylhydroxylamine;N,N-diethylhydroxylamine; N,N- dipropylhydroxylamine; N,Ndiisopropylhydroxylamine; N,N-diarnylhydroxylamine;N,N-n-propylmethylhydroxylamine; N,N-ethylmethylhydroxylamine;N,N-butylmethylhydroxylamine; N,N-dihexylhydroxylamine and the like.Likewise, it will be understood that the N,N-dialkylhydroxylamine saltsof the invention may be employed with vinyl compounds such as styrene,divinylbenzene, methylstyrene, ring methylated styrene,ethylvinylbenzene, vinylnaphthalene, ring chlorinated styrenes, etc.,although styrene will be used throughout the specification to illustratethe invention.

The N,N-di-lower alkylhydroxylamine salts useful in the stabilizingprocess of this invention are novel compounds and the mono salts willhave the structural forwhere R R R and n are defined above and R and Rare alkyl groups containing from 1 to 6 carbon atoms. Examples of theacid salts of the N,N-di-lower alkylhydroxylamines include the lactates,tartrates, citrates, malates, malonates, succinates, glutarates,adipates, pimelates, sebacates, azelates, and the like. It will beunderstood that the operable salts will include those containing one ormore moles of N,N-dialkylhydroxylamine per mole of polycarboxylic acid.

When used in the process of the invention to inhibit the prematurepolymerization of the vinyl compounds, the compounds will be usedpreferably at a concentration of about 0.1 to 2% by weight of the vinylcompound. However, it is to be understood that lower concentrations mayalso be employed, as well as higher concentrations, and the generaloperable range will vary from between about 0.001 to 5% by weight of thevinyl compounds to be stabilized.

The salts are prepared readily by the addition of an organic solventsolution of the di-lower alkylhydroxylamine to an organic solventsolution of the carboxylic acid, although in some instances,particularly with the higher molecular weight acids, highly concentratedaqueous solutions of the hydroxylamine are simply mixed with the acid.The resulting salt which forms, may be filtered off if it precipitatesfrom solution, or it may be isolated by the usual techniques of organicchemistry. The following examples illustrate the preparation of thesenovel N,N-di-lower alkylhydroxylamine salts:

Example 1.-Diethylhydroxylammonium citrate A solution of 5.25 g. (0.025mole) of citric acid in 250 ml. of ethyl ether plus 50 ml. of isopropylalcohol was prepared and to this a solution of 7.60 g. of 88%diethylhydroxylamine (diethylhydroxylamine content of 6.70.

Example 2.Bis(diethylhydroxylammonium) tartrate Fifteen grams (0.1 mole)of tartaric acid was added to 19.6 g. (0.2 mole) of 93%diethylhydroxylamine. The addition of 5 ml. of water was necessary tocompletely etfect solution. This was heated and chilled to cause theseparation of a yellow waxy solid. After two recrystallizations fromisopropyl alcohol the melting point was 111- 114 C. and the productweighed 16.6 g. This analyzed 8.86% nitrogen and 57.60%diethylhydroxylamine. The theoretical values are 8.54% nitrogen and54.20% diethylhydroxylamine.

Example 3.-Bis (diethylhydroxylammonium) adipate At room temperature,9.2 g. (0.1 mole) of 92.8% diethylhydroxylamine was stirred with 7.3 g.(0.05 mole) of adipic acid. On warming slightly, a homogeneous darkamber oil was obtained. This product was soluble in acetone, diacetonealcohol, xylene and water.

Example 4.-Diethylhydroxylammonium lactate At room temperature, 10.6 g.(0.1 mole) of 85% lactic acid was dissolved in 9.6 g. (0.1 mole) of 93%diethylhydroxylamine. The resulting solution was warmed and then placedin a vacuum dessicator over P for 5 days. The liquid product weighed 138g. (77% yield).

Example 5 .Bis(diethylhydroxylammonium) succinate A mixture of 9.2 g.(0.1 mole) of 92.6% diethylhydroxylamine and 5.9 g. (0.005 mole) ofsuccinic acid was stirred and warmed until the acid was dissolved. Theproduct was an orange oil which was soluble in water, xylene, acetoneand diacetone alcohol.

In using the above salts as polymerization inhibitors they may bedissolved in uninhibited vinyl monomer to prepare a concentratecontaining 5 to of inhibitor, aliquots of which are then used as desiredby adding to the monomer to be stabilized. Such techniques make for easeof handling and are conducive to a highly efficient stabilizingprocedure.

In order to further illustrate the etfectiveness of the invention, thefollowing examples are given:

Example 6 The polymerization of styrene containingN,N-dialkylhydroxylamine and a control of styrene containing noinhibitor was studied simultaneously by means of dilatometricexperiments. The dilatometer employed was of the stopcock type with abulk of 25 ml. capacity and an 80 cm. capillary stem of 2.0 mm. bore.The open capillary top was plugged with a small glass bead to eliminateany evaporation of the monomer. In order to insure that it was free ofinhibitor and any impurities, the styrene employed was distilled andmaintained under refrigeration prior to polymerization. The styrenecharge was drawn into the dilatometer by suction through a tubeextending below the bottom of the styrene. The dilatometer was thenplaced in a mineral oil bath so that the bulb and 5 cm. of the capillarystem were submerged below the oil level. The oil bath was then heated toraise the temperature of the styrene to 120:0.2 C. Approximately minuteswere required for the solution to reach thermal equilibrium uponimmersion of the dilatometer in the thermostatically-controlled bath.

Initially upon heating, the volume of the styrene in the capillary tuberises to a given level. Thereafter, as polymerization begins andcontinues, the level of the liquid in the capillary tube progressivelyfalls. Thus, the decrease in volume of the liquid in the capillary tubeprovides a direct measure of the amount of polymerization which isoccurring, the smaller the decrease, the greater the inhibition. Thefollowing tables indicate the compounds tested and the results obtained.

TABLE I.--STABILIZATION EFFECTS OF 0.5% BY WEIGHTOINI,N-DIETIIYLHYDROXYLAMINE SALTS ON STY- R Decrease in height in 2hrs. at C.

Cm. Percent of control A Control 26. 5 Diethylhydroxylamina. 4. 2 15. 8Diethylhydroxylamine sulfate" 12.0 43. 0 Diethylhydroxylam inebistartrate. 2. 2 8. 3 Dicthylhydroxylamine bis-adipate 1. 4 5. d

B Control 26. 7 Diethylhydroxylamine. 2. 4 9. 6 Diethylhydroxylamineb 1. 2 4. 5 Diethylhydroxylarnine lactate 0.8 3. 0 Diethylhydroxylamineoxalate 3. 5 13. 0 Diethylhydroxylamine malcate 23. 6 88. 4

0 Control 39. 8 Diethylhydroxylamine 6. 2 15. 6 Diethylhydroxylaminehydrochlor 7. 0 l7. 6 Diethylhydroxylamine benzoate 30. O 75. 4Diethylhydroxylamine salicylate 18.2 45. 7 Diethylhydroxylamine citrate2. 5 6. 3

It is evident from the above Table I that the acid salts of N,N-di-loweralkylhydroxylamines are markedly superior polymerization inhibitors thanthe diethylhydroxylamine, its hydrochloride, its aromatic carboxylicacid salts, or the oxalates and maleates.

Example 7 The evaluation procedure of Example 1 was repeated usingbis-dibutylhydroxylamine sebacate at a concentration of 0.5% by weightof the styrene. The polymerization of the styrene was greatly inhibited.

It will be understood that numerous changes and variations may be madefrom the above description and examples without departing from thespirit and scope of the invention.

I claim:

1. The process of stabilizing an aromatic vinyl compound againstpolymerization which comprises incorporating in said vinyl compound apolymerization inhibiting amount of an N,N-di-lower alkylhydroxylaminesalt of hydroxy acids and polycarboxylic acids which have the formula mR1C|H-(|3COOI-I R2 \R2 /n where R is a member selected from the groupconsisting of methyl and carboxyl, R is a member selected from the groupof hydrogen and hydroxyl, R is a member selected from the groupconsisting of hydrogen and carboxyl, n is an integer from 0 to 8, andwith the proviso that when R is methyl, at least one R is hydroxyl.

2. The process of stabilizing styrene against polymerization whichcomprises incorporating in said styrene from .001% to 5.0% by weight ofstyrene an N,N-dilower alkylhydroxylamine salt of an acid as defined inclaim 1.

3. The process of claim 2 wherein the stabilizer isN,N-di-ethylhydroxylamine lactate.

4. The process of claim 2 wherein the stabilizer isN,N-diethylhydroxylamine tartrate.

5. The process of claim 2 wherein the N,N-diethylhydroxylamine citrate.

6. The process of claim 2 wherein the stabilizer isN,N-diethylhydroxylamine adipate.

7. The process of claim 2 wherein the N,N-diethylhydroxylaminesuccinate.

8. A composition comprising an aromatic vinyl compound containing apolymerization inhibiting amount of an N,N-di-lower alkylhydroxylaminesalt of an acid as defined in claim 1.

9. A composition comprising styrene and an amount stabilizer isstabilizer is 5 of N,N-diethylhydroxylamine lactate sufficient toinhibit polymerization.

10. A composition comprising styrene and an amount ofN,N-diethylhydroxylarnine tartrate sufficient to inhibit polymerization.

11. A composition comprising styrene and an amount ofN,N-diethylhydroxylamine citrate sufiicient to inhibit polymerization.

12. A composition comprising styrene and an amount ofN,N-diethylhydroxylamine adipate sufiicient to inhibit polymerization.

13. A composition comprising styrene and an amount ofN,N-diethylhydroxylamine succinate suflicient to inhibit polymerization.

References Cited UNITED STATES PATENTS 2,302,749 11/1942 Dean 2605012,830,033 4/1958 Beaver 260--666.5 X 2,965,685 12/1960 Campbell260-666.5

l0 DELBERT E. GANTZ, Primary Examiner.

G. E. SCHMITKONS, Assistant Examiner.

